1. Field of the Invention
The present invention relates to a hydraulic circuit of an injection molding machine having a differential mode in which drain oil ejected from an injection cylinder during injection is utilized as working fluid.
2. Description of the Relevant Art
Japanese Patent Application Laid-Open (kokai) No. 6 (1994)-246800 discloses a conventional hydraulic circuit of an injection molding machine which permits selection of either a regular mode or a differential mode. In the regular mode, all drain oil ejected from a front oil chamber of an injection cylinder during injection is returned to an oil tank. In the differential mode, all or a portion of drain oil is supplied to a rear oil chamber of the injection cylinder. FIG. 4 schematically shows a simplified block diagram of a conventional hydraulic circuit 50 for the purpose of illustrating the principle of the hydraulic circuit 50.
In FIG. 4, when a mode selector valve 51 is changed over to a right-side position marked with symbol R (hereinafter referred to as the R position), the regular mode is established. Working fluid discharged from a hydraulic power unit 52 during injection is supplied via a main circuit 53 to a rear oil chamber 61r of an injection cylinder 61 of an injection molding machine 60. All drain oil ejected from a front oil chamber 61f is returned to an oil tank 54 via the mode selector valve 51 set in the R position and the main circuit 53.
When the mode selector valve 51 is changed over to a left-side position marked with symbol L (hereinafter referred to as the L position), a differential mode is established. Working fluid discharged from the hydraulic power unit 52 during injection is supplied to the rear oil chamber 61r of the injection cylinder 61 via the main circuit 53. Drain oil ejected from the front oil chamber 61f flows through the mode selector valve 51 set in the L position and merges with working fluid from the main circuit 53. The resultant mixture is supplied to the rear oil chamber 61r of the injection cylinder 61. In the differential mode, drain oil ejected from the injection cylinder 61 is utilized as working fluid without being returned to the oil tank 54, thereby enabling injection at a relatively high flow rate of working fluid without an increase in the amount of working fluid supplied from the hydraulic power unit 52, and thus implementing an increase in injection speed, a reduction in the size of an oil line, and prevention of impact pressure.
However, in spite of yielding of the above advantages through selection of the differential mode, the hydraulic circuit 50 having the differential mode yields the following disadvantage. Even when the supply of working fluid from the main circuit 53 is stopped, drain oil ejected from the front oil chamber 61f circulates directly to the rear oil chamber 61r due to inertia of a piston of the injection cylinder 61. As a result, at the time of stoppage of injection, braking control cannot be performed precisely and accurately. For precise, accurate braking, the conventional hydraulic circuit requires an additional braking circuit, resulting in an increase in the number of components of the hydraulic circuit and an associated increase in manufacturing cost. Also, control becomes complicated and unstable.